1. Field of the Invention
The present invention relates to a halftone dot generator, and more particularly to a technique for recording a halftone dot image with a plurality of grained screen patterns.
2. Description of Background Art
As well known in the art, an ordinary halftone dot generator generates a screen pattern having a single peak of threshold values. The screen pattern is applied to every halftone dot, and therefore, a periodic distribution in density such as a moire often appears on a reproduced halftone dot image.
In order to prevent or suppress the periodic distribution, a grained screen pattern is often used in image reproduction. However, even if a grained screen pattern is employed, the periodic distribution in density cannot be perfectly suppressed on a uniform color region of the reproduced image, such as a region of a blue sky. This is because the grained screen pattern is used in common to all areas arrayed at a screen pitch. Although the periodic distribution can be further suppressed when the grained screen pattern is so constructed that the screen pitch is longer than the size of the uniform color region, such a grained screen pattern has large amount of data, and a screen pattern memory having large capacity should be prepared.